Peak rectifier circuits change AC into pulsed DC by eliminating the negative half-cycles or alternating the AC voltage. Only a series of pulsations of positive polarity remain. Conventional rectifiers are classified according to the number of AC conducted positive half-cycles (i.e. half-wave rectifiers or full-wave rectifiers). Two types of circuits are used for full-wave, single-phase rectification: One circuit uses a transformer with a mid-tap in the secondary winding and a pair of rectifier diodes; the other uses a bridge diode configuration which requires two extra rectifier diodes and, in case of employ a transformer after the AC input voltage, the secondary requires only half as much winding. Performances are similar except that the bridge diodes are subjected to only half the peak inverse voltage of the center-tap circuit.
As both halves of the cycle are rectified, the current and voltage on the input side are normal effective values; RMS values on the output side are the same as for a sine wave while the DC or average values are twice that of a half-wave circuit. A Fourier analysis of the rectifier output waveform yields a constant term (DC voltage) and a series of harmonic terms. Filters are usually added to extract the constant term and attenuate harmonic terms. Inductor-input filters are preferred in higher-power applications in order to avoid excessive turn-on and repetitive surge currents. However, the use of an inductor alone is generally impractical, particularly when variable loads must be handled because the attenuation is not sufficient with reasonable values of inductance. When capacitor-input filters are used, diodes whose average rating more nearly matches the load requirement can be used if a source-to-load resistance ratio of about 0.03 and voltage regulation of about 10% are acceptable. A capacitor-input filter has a shunt capacitor presented to the rectifier output. Each time the positive peak alternating voltage is applied to one of the rectifier anodes, the input capacitor charges up to just slightly less than this peak voltage.
No current is delivered to the filter until another anode approaches its peak positive potential. When the capacitor is not being charged, its voltage drops off nearly linearly with time because the load draws a substantially constant current. Use of an input capacitor increases the average voltage across the output terminals of the rectifier and reduces the amplitude of the ripple in the rectifier output voltage. In any case, the capacitor charges up to the peak voltage of the rectifier output during the time that current pulses are delivered to the filter load. If the capacitance is large, more energy is stored during current pulses and the capacitor output voltage remains relatively high during discharge. On the other hand, if the load current is large, the capacitor discharges rapidly between current pulses and the average DC output voltage falls to a low value. This continuous charge-discharge cycle stress imposed on the filter capacitor contributes to the aging of the device and eventually to its failure. Replacing capacitors periodically is the only way to insure a very high Mean Time Before Failure (MTBF) for capacitors.
DC Aluminum electrolytic capacitors use an aluminum oxide layer as the dielectric and a dielectric grade aluminum foil as the current input bias. Both the materials and the processing have non-uniformities on a small scale. Two elementary mechanisms lead to capacitor field aging. The first is due to the leakage currents and the second one has to do with physical conditions such combinations of heat and chemical contaminant.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications and variations thereof will be apparent to those skilled in the art.